It is well-known that acrylonitrile polymer and copolymer fibres which are produced by a wet-spinning process exist for a time during the manufacturing process in a gel form. In this form they have a very open structure which enables them to be dyed much more quickly and at much lower temperatures than would be the case for a fully processed and stabilised fibre. When dyeing this gel form it is often advantageous to apply the dyes from highly polar solvents, such as dimethyl acetamide, dimethyl formamide, dimethyl sulphoxide or ethylene glycol carbonate which are solvents or partial solvents for the acrylic fibre. Conventional basic dyes, whilst exhibiting good affinity for the acrylonitrile polymer and copolymer fibre, often have low solubility in some of these solvents which limits their use in such a dyeing process. We have found that certain modifications to the structure of these basic dyes improve their solubility in such solvents. In addition these modifications render the dyes virtually insoluble in water and we have found this to be beneficial in reducing the extent to which dye is washed off the dyed fibre in the aqueous rinsing processes which usually follow the dye application process.